But she was quickly forced into prostitution, starting in Guadalajara, one of Mexico's largest cities, followed by several other cities.

She said: "I started at 10am and finished at midnight. We were in Guadalajara for a week. Do the maths. 20 per day for a week.

"Some men would laugh at me because I was crying. I had to close my eyes so that that I wouldn't see what they were doing to me, so that I wouldn't feel anything."

Her life became a blur of brothels, motels, streets and homes, with no days off, racking up at least 30 men a day, every day of the week.

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Karla's first insight into what would be four years of her life was in Guadalajara

One time her trafficker gave her a love bite which prompted a horrendous beating.

She said: "He started beating me with a chain in all of my body. He punched me with his fists, he kicked me, pulled my hair, spit at me in the face, and that day was when he also burned me with the iron. I told him I wanted to leave and he was accusing me of falling in love with a customer. He told me I liked being a whore."

Even when police raided a hotel known for prostitution the horror wasn't over.

Instead of rescuing the girls, some of the 30 police officers took them to rooms and started filming them in compromising positions before telling the girls the videos would be sent to their families if they refused to do anything they asked.

When she gave birth aged 15, the pimp father of her child said he would kill the baby if she did not do everything he wanted.

She was only allowed to have her daughter for a month before he took her away and was only allowed to see the girl when she was more than a year old.

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Karla was human trafficked at the young age of 12

After four excruciatingly long and frightening years she was at last rescued in 2008 during an anti-trafficking operation in Mexico City.

Despite a life of scars, she has met the Pope and US Congress as she uses her story to support the development of a law called HR 515 - or Megan's Law - which means US authorities have to share information about American child sex offenders when they try to travel abroad.

Karla warned: "These minors are being abducted, lured, and yanked away from their families. Don't just listen to me. You need to learn about what happened to me and take the blindfold off your eyes."